I have an old vitamix( 35+ years old) with the stainless blender jar. I'd would guess the only two advantages would be that if you use it to grind spices like cumin seed it wouldn't scratch up like the plastic jar, the other being that would you want to use it to make a hot soup you aren't running the risk of picking up chemicals from a heated plastic container.
I can't compare how effective it is because the stainless jar is square in cross section vs a more round geometry for the modern plastic jars. The Action Dome top, which is open all the time is more bother than I think it's worth. I find myself using a piece of plastic wrap between it and the jar because the clips aren't strong enough to seal for liquids even though it has a new gasket. I should point out that this blender was a gift just a few years ago, so I'm not the original owner, nor have I had it all that time. I haven't found the need to order a plastic replacement jar from Vita Mix, if they're still available, as the cost was over $100. I have been looking for a replacement lid that was less bother, but haven't found the correct size.
I replaced the spout that was original to the jar with a food safe washer and stainless bolt because it required completely dismantling it to clean it. This would not be an issue if you were buying a Waring jar. I find cleaning this jar more work and less satisfactory than a convential design that allows removal of the bottom. Again, the new ones may do a better job of self cleaning because of the shape.
The person who gave me this machine said the motor is much more powerful than the current versions. I have no experience with the new ones to comment, other than it's loud enough that I keep a pair of ear protector earmuffs on the machince for when I use it.
Depending on what you use it for you may prefer the stainless Waring jar. I use mine to make treats from frozen bananas, or blended sauces and occasionally still use it to grind large amounts of spices or grains that aren't recommended for the grain mill I have.